There is known a magnetic random access memory (hereinafter, referred to as an MRAM) which stores data by controlling a magnetization direction of a memory cell. There are several kinds of MRAMs depending on a method for storing the magnetization direction.
In a cited document (U.S. Pat. No. 6,545,906B), a technique of a toggle-type magnetic random access memory (hereinafter, referred to as a toggle MRAM) is disclosed. The toggle MRAM has a memory element which is a tunnel magneto-resistive element using a laminated ferri structure in a free layer. This MRAM has excellent selectivity of the memory cell at the time of a write operation, and is characterized by a point that there is almost no multiple write to occur. Details will be explained below.
FIG. 1 is a sectional view showing a structure of a conventional toggle MRAM. A magneto-resistive element 105 of a memory cell 110 in the MRMA includes an antiferromagnetic layer 104, a laminated ferri-fixed layer 103, a tunnel insulating layer 102, and a laminated ferri-free layer 101, which are laminated in this order. The laminated ferri fixed layer 103 has a laminated ferri structure, and includes a ferromagnetic layer 116, a non-ferromagnetic layer 115, and a ferromagnetic layer 114. The laminated ferri-free layer 101 has a laminated ferri structure, and includes a ferromagnetic layer 113, a non-ferromagnetic layer 112, and a ferromagnetic layer 111. The magneto-resistive element 105 is held between a write word line 126 and a bit line 127 that are mutually intersected in a substantially perpendicular state.
The laminated ferri-fixed layer 103 has a laminated ferri structure so as not to generate a magnetic field from the laminated ferri-fixed layer 103. A magnetization direction of the laminated ferri-fixed layer 103 is fixed by the antiferromagnetic layer 104. The laminated ferri-free layer 101 also has a laminated ferri structure, and the magnetic field is not generated from the laminated ferri-fixed layer 103 and the laminated ferri-free layer 101 as long as there is no external magnetic field to be applied.
FIG. 2 is an upper surface view showing a structure of a conventional toggle MRAM. Plural numbers of the write word lines 126 and the bit lines 127 are orthogonally placed (one of each is shown in FIG. 2), and the magneto-resistive element 105 is placed in each of cross points. The magneto-resistive element 105 has an easy magnetization direction (or an easy magnetization axis) which is directed at substantially 45 degrees (θ) relative to the word line 126 and the bit line 127. It is due to consideration for easiness of a toggle operation.
Next, a principle of a write operation of the conventional toggle MRAM will be explained. In the case of the toggle MRAM, data can be exclusively written from “1” to “0” or from “0” to “1”. That is, it is impossible to overwrite data of “1” to “1” and “0” to “0”. Therefore, a write operation of the toggle MRAM is performed by reading a selected memory cell in advance to determine whether or not a magnetization direction is changed (or whether or not toggle operation is performed) in first and second free layers based on the read information and information to write. Specifically, if the read information (“0” or “1”) is equal to the information to write (“0” or “1”), a toggle operation is not performed, while a toggle operation is performed if a read operation is different from the information to write.
FIGS. 3A to 3H are views showing a principle of a toggle operation in the toggle MRAM of the conventional technique. FIG. 3A is a timing chart of a write current IBL flowing in the bit line 127. FIG. 3B is a timing chart of a write current IWL flowing in the word line 126. FIG. 3C is a time variation in a magnetization direction 121s of the ferromagnetic layer 113 and a magnetization direction 122s of the ferromagnetic layer 111 in a selected cell as the memory cell 110 into which data is written. FIG. 3D is a time variation in a direction of a magnetic field generated by the write current IBL and the write current IWL. FIG. 3E is a time variation in a magnetization direction 121a of the ferromagnetic layer 113 and a magnetization direction 122a of the ferromagnetic layer 111 in a non-selected cell placed in the same bit line 127 with the selected cell. FIG. 3F is a time variation in a direction of the magnetic field generated by the write current IBL. FIG. 3G is a time variation in a magnetization direction 121b of the ferromagnetic layer 113 and a magnetization direction 122b of the ferromagnetic layer 111 in a non-selected cell placed in the same word line 126 with the selected cell. FIG. 3H is a time variation in a direction of the magnetic field generated by the write current IWL.
Referring to FIG. 3a, in the toggle operation, the write current IBL is supplied to the bit line 127 at time t2. The write current IWL is supplied to the word line 126 at time t3. The write current IBL is suspended at time t4. The write current IWL is suspended at time t5. Due to a series of the above current controls, a rotational magnetic field starting from a magnetic field 123, through a magnetic field 124, to a magnetic field 125 as shown in FIG. 3(d) is added to the selected cell in a cross point between the selected word line 126 provided with the write current IWL and the selected bit line 127 provided with the write current IBL. Therefore, the magnetization direction 121s of the ferromagnetic layer 113 and the magnetization direction 122s of the ferromagnetic layer Ill in the selected cell are rotated (or changed) as shown in FIG. 3(c), so that data can be written. That is, data is rewritten (or toggled) into a state of “1” if an initial state is a state of “0”, and into a state of “0” if the initial state is a state of “1”.
At this time, a unidirectional magnetic field such as the magnetic field 123 shown in FIG. 3(f) is exclusively added to the non-selected cell in the same bit line 127 as the selected cell. Therefore, the magnetization direction 121a of the ferromagnetic layer 113 and the magnetization direction 122a of the ferromagnetic layer 111 in the non-selected cell are returned to an original state with some fluctuations as shown in FIG. 3(e), so that data is not written. Similarly, a unidirectional magnetic field such as the magnetic field 125 as shown in FIG. 3(h) is exclusively added to the non-selected cell placed in the same word line 126 as the selected cell. Therefore, the magnetization direction 121b of the ferromagnetic layer 113 and the magnetization direction 122b of the ferromagnetic layer 111 in the non-selected cell are returned to the original state with some fluctuations as shown in FIG. 3(g), so that data is not written.
FIG. 4 is a view showing a state of magnetization directions of ferromagnetic layers in an upper layer and a lower layer receiving thermal disturbance. In the case of the toggle MRAM, composite magnetization of the laminated ferri-free layer 101 approaches saturation in accordance with the increase of an applied magnetic field due to the increase of a flop magnetic field. In this case, there is a possibility of switching between magnetization of the ferromagnetic layer 111 in the upper layer and magnetization of the ferromagnetic layer 113 in a lower layer in the laminated ferri-free layer 101 caused by the thermal disturbance. There is a demand for a technique which uses a write magnetic field to prevent composite magnetization from approaching saturation and to suppress a possibility of switching between magnetization of the ferromagnetic layer in the upper layer and magnetization of the ferromagnetic layer in the lower layer due to the thermal disturbance. Here, it is assumed that the flop magnetic field is the magnetic field in a boundary between a region with a nonlinear change and a region with a linear change in magnetization of the laminated ferri-free layer 101 with respect to the magnetic field.
As a related technique, Japanese Laid Open Patent Application JP-P2004-158766A discloses a technique of a magneto-resistive effect element and a magnetic memory device. The magneto-resistive effect element has a first magnetization fixed layer, a first tunnel barrier layer, a first ferromagnetic layer, a magnetization free layer including a nonmagnetic layer and a second ferromagnetic layer, a second tunnel barrier layer, and a second magnetization fixed layer. The first and second magnetization fixed layers have magnetization directions that are opposite from each other, The first ferromagnetic layer and the second ferromagnetic layer are antiferromagnetically coupled via the nonmagnetic layer. One of the first and second ferromagnetic layers has magnetization which is larger than magnetization of the other one. One of the first and second magnetization fixed layers has magnetization which is larger than the magnetization of the other one. The magnetization fixed layer with larger magnetization is formed in a side closer to the ferromagnetic layer with smaller magnetization among the first and second ferromagnetic layers.
As a related technique, Japanese Laid Open Patent Application JP-P2003-298023A discloses a technique of a magnetic memory and a magnetic memory device. The magnetic memory includes first and second magneto-resistive effect elements to be mutually opposed, a common wiring interposed between the first and second magneto-resistive effect elements, a first wiring which intersects the common wiring by holding the first magneto-resistive effect element therebetween, and a second wiring which intersects the common wiring by holding the second magneto-resistive effect element therebetween. The first magneto-resistive effect element has a first pin layer and a first free layer. The first pin layer includes a laminated substance of laminating even-numbered ferromagnetic layers via the nonmagnetic layer, and maintains a magnetization direction at the time of applying a first magnetic field which is generated by causing a write current to flow in the common wiring and the first wiring. The first free layer includes a laminated substance which is interposed between the first pin layer and the common wiring and laminates one ferromagnetic layer or a plurality of ferromagnetic layers via the nonmagnetic layer, and has a reversible magnetization direction when the first magnetic field is applied. The second magneto-resistive effect element has a second pin layer and a second free layer. The second pin layer includes a laminated substance of laminating one ferromagnetic layer or odd-numbered ferromagnetic layers of three or more via the nonmagnetic layer, and maintains a magnetization direction at the time of applying a second magnetic field which is generated by causing a write current to flow in the common wiring and the second wiring. The second free layer includes a laminated substance which is interposed between the second pin layer and the common wiring and laminates one ferromagnetic layer or a plurality of ferromagnetic layers via the nonmagnetic layer, and has a reversible magnetization direction when the second magnetic field is applied. The number of the ferromagnetic layers included in the first free layer and the number of the ferromagnetic layers included in the second free layer are odd numbers in both or even numbers in both.
As a related technique, Japanese Laid Open Patent Application JP-P2003-110164A discloses a technique of a magneto-resistive effect element, a magnetic memory and a magnetic head. The magneto-resistive effect element has a magnetic laminated film, a ferromagnetic substance film, and an insulating film arranged between the magnetic laminated film and the ferromagnetic substance film. Disclosed is the magneto-resistive effect element of a tunnel junction type, in which a current is made to flow between the magnetic laminated film and the ferromagnetic substance film by tunneling the insulating film. The magnetic laminated film has a first ferromagnetic substance layer, a second ferromagnetic substance layer, and an antiferromagnetic substance layer inserted between the first and second ferromagnetic substance layers.
As a related technique, Japanese Laid Open Patent Application JP-P2002-353535A discloses a technique of a magneto-resistive effect element, a magneto-resistive effect type magnetic sensor, a magneto-resistive effect type magnetic head, and a magnetic memory. The magneto-resistive effect element has a laminated structure unit of laminating at least a free layer with magnetization to be rotated in accordance with the external magnetic field, a fixed layer, an antiferromagnetic layer for fixing magnetization of the fixed layer, and a nonmagnetic layer interposed between the free layer and the fixed layer. Disclosed is a giant magneto-resistive effect element, in which a substantial lamination direction of the laminated structure unit is made to be an energizing direction of a sense current. Arranged in the laminated structure unit is an energizing control layer to dispersedly form micro energized regions across a path of the sense current.
As a related technique, Japanese Laid Open Patent Application JP-P2002-151758A discloses a technique of a ferromagnetic tunnel magneto-resistive effect element, a magnetic memory, and a magneto-resistive effect type head. The ferromagnetic tunnel magneto-resistive effect element has a tunnel barrier layer which is formed between a first magnetic layer and a multi-layer structure of laminating at least five or more layers including a ferromagnetic layer and an intermediate layer. The first magnetic layer has a magnetization direction restricted to an acting external magnetic field. The ferromagnetic layer to compose the multi-layer structure has a magnetization direction rotated to the external magnetic field, in which magnetization is antiferromagnetically arranged via the intermediate layer. The ferromagnetic tunnel magneto-resistive effect element has a ferromagnetic tunnel magneto-resistive effect film having resistance changed by a relative angle of magnetization in the first magnetic layer and the ferromagnetic layers to compose the multi-layer structure, lower and upper electrodes to be electrically connected to lower and upper magnetic layers in order to provide a sense current for the ferromagnetic tunnel magneto-resistive effect film, and a detection means adapted to detect a resistance change.